1. Field of the Invention
The present invention relates to an optical modulator and a communications system including the optical modulator. More particularly, the present invention relates to an optical modulator for use to transmit an RF signal having a frequency of several GHz or more by a lightwave communications technique and also relates to a communications system including such an optical modulator.
2. Description of the Related Art
A system for exchanging or processing information by using an optical signal needs to modulate the phase or intensity of light by means of an electric signal (e.g., an RF signal falling within the microwave or milliwave band). Light can be modulated for that purpose either by a direct modulation technique or by an external modulation technique.
The direct modulation technique is a method of changing the intensity of light that has been emitted from a light source (e.g., a semiconductor laser diode) by directly changing the amount of drive current being supplied to the light source as shown in FIG. 1A. The direct modulation technique contributes to reducing the overall size of a communications system because no modulators need to be provided outside of the light source. According to this method, however, it is difficult to modulate the light at a high frequency of several GHz or more. In addition, long distance fiber optics transmission can be carried out only under limited conditions due to a chirping phenomenon which is often observed in semiconductor laser radiation.
In the external modulation technique on the other hand, light that has been emitted from a light source such as a semiconductor laser diode (i.e., light with a stabilized output power) is input to an optical modulator, which modulates the phase or intensity of the light as shown in FIG. 1B. In this technique, the light may be modulated by utilizing electro-optical effects, acoustooptical effects, magnetooptical effects or nonlinear optical effects.
As described above, it is difficult to achieve ultrahigh speed light modulation by the method of directly modulating the output of a semiconductor laser diode. Thus, an external modulator is currently under vigorous research and development because an element of that type normally achieves high speed light modulation. Among various types of external modulators, an electro-optical modulator, which uses dielectric crystals exhibiting Pockel""s effect, can operate at such an extremely high speed and yet causes little disturbance in phase as a result of the modulation. For that reason, this electro-optical modulator can be used very effectively in high-speed data transmission, long distance fiber-optics communications and other applications. Also, if an optical waveguide structure is constructed using such an electro-optical modulator, the modulator may be implemented at a small size and can operate efficiently enough at the same time.
An electro-optical modulator usually includes: a transmission line, which is provided as a modulating electrode (or signal electrode) on electro-optic crystals to propagate a modulating signal therethrough; and an optical waveguide, which is provided near the transmission line. In this electro-optical modulator, the refractive index of the optical waveguide is changed by an electric field to be induced around the modulating electrode, thereby modulating the phase of the light wave being propagated through the optical waveguide.
Crystals that are normally used in such an electro-optical modulator have a relatively small electro-optic coefficient. The electro-optic coefficient is a parameter that forms the basis of optical modulation. Accordingly, an electro-optical modulator should apply an electric field to the optical waveguide as efficiently as possible.
FIG. 2 is a cross-sectional view showing the fundamental structure of an electro-optical modulator. As shown in FIG. 2, an optical waveguide is provided on the surface of a substrate that is made up of crystals exhibiting electro-optical effects (i.e., electro-optic crystals), and a modulating electrode is provided on the optical waveguide.
The electro-optic crystals have optical anisotropy and change their refractive indices substantially proportionally to the strength of the electric field applied thereto (i.e., exhibit the Pockel""s effect). Thus, by adjusting the potential V applied to the modulating electrode, the refractive index n of the optical waveguide can be changed. The variation xcex94n in the refractive index of the optical waveguide is proportional to the strength of the electric field E applied to the optical waveguide. When the refractive index of the optical waveguide changes by xcex94n, the phase of the output light shifts by xcex94xcfx86 as shown in FIG. 2. The phase shift xcex94xcfx86 is normally proportional to the product of the strength of the electric field E and the length L of the optical waveguide.
To create the electric field in the optical waveguide, a modulating signal is supplied externally (i.e., from outside of the optical modulator) to the electrode of the optical modulator by way of the input line. Thus, it is important to input the modulating signal as efficiently as possible.
Next, a specific configuration for a conventional optical modulator will be described in further detail with reference to FIG. 3. FIG. 3 is a plan view of a conventional optical modulator as disclosed in U.S. Pat. No. 5,400,416.
As shown in FIG. 3, the optical modulator includes a substrate 101, which is made of an electro-optic material, and an optical waveguide 112, which may be formed on the surface of the substrate 101 by thermally diffusing a metal element toward a portion of the substrate 101, for example.
On the surface of the substrate 101, a parallel coupled line structure 113, obtained by patterning a metal film of aluminum, gold or other suitable metallic material, is provided on the right- and left-hand sides of the optical waveguide 112. On the other hand, a ground plane 114, also obtained by patterning a metal film, is provided on the back surface of the substrate 101. The parallel coupled line structure 113 includes two lines 113a and 113b that extend parallelly to each other.
In the example illustrated in FIG. 3, the two lines 113a and 113b of the parallel coupled line structure 113 are coupled together by way of a single line 124. However, the U.S. Pat. No. 5,400,416 identified above also discloses a structure in which the two lines 113a and 113b are not coupled together.
An input terminal 129 is further provided so as to be connected to a portion of the line 113b by way of a tap 128. An RF signal source 119 is connected between the input terminal 129 and the ground plane 114.
Incoming light is introduced through one end of the optical waveguide 112, passed through a portion of the optical waveguide 112 in the gap 116 between the two lines 113a and 113b of the parallel coupled line structure 113, and then output as outgoing light through the other end of the optical waveguide 112. In the meantime, the input terminal 129 and the parallel coupled line structure 113 are magnetically coupled together. Thus, an RF signal, supplied from the RF signal source 119, is propagated through the respective lines 113a and 113b of the parallel coupled line structure 113 to generate an electric field in the gap 116 between the lines 113a and 113b. According to the strength of that electric field, the refractive index of the optical waveguide 112 changes due to the electro-optical effects. As a result, the phase of the outgoing light is modulated. In this manner, the present optical modulator can operate as a phase modulator.
The parallel coupled line structure normally operates in either even mode or odd mode. In the odd mode, the voltages of the two lines included in the parallel coupled line structure have mutually opposite polarities, thus inducing a huge electric field in the gap between them. The optical modulator shown in FIG. 3 achieves light modulation highly efficiently by operating the two lines 113a and 113b of the parallel coupled line structure 113 in the odd mode responsive to the modulating signal.
However, to use such an optical modulator much more extensively in an optical communications system, for example, the performance of the optical modulator is not yet fully satisfactory but is still to be improved in many respects. That is to say, the development of an even more efficient optical modulator is awaited.
In order to overcome the problems described above, preferred embodiments of the present invention provide a highly efficient optical modulator for use effectively in an optical communications system, for example.
An optical modulator according to a preferred embodiment of the present invention preferably includes an optical waveguide, a modulating electrode, a conductive layer, an electric signal input section, and connector members. At least a portion of the optical waveguide is preferably made of an electro-optic material. The modulating electrode preferably includes a first conductor line and a second conductor line, which are coupled together electromagnetically, and preferably applies a modulating electric field to a portion of the optical waveguide. The conductive layer preferably forms a first microstrip line with the first conductor line and a second microstrip line with the second conductor line, respectively. Through the electric signal input section, an RF modulating signal is preferably supplied to the modulating electrode. The connector members preferably connect the first and second conductor lines together at both ends thereof. In this optical modulator, the first and second conductor lines preferably function as an odd-mode resonator for the RF modulating signal.
In one preferred embodiment of the present invention, the optical waveguide preferably includes: at least two optical waveguide branches; an optical input portion, which combines the two branches together; and an optical output portion, which also combines the two branches together. The portion of the optical waveguide, to which the modulating electric field is applied, is preferably divided into the two optical waveguide branches. The modulating electrode is preferably provided so as to apply electric fields with mutually opposite polarities to the two optical waveguide branches, respectively, and preferably functions as an intensity modulator for modulating the intensity of light that has been input to the optical waveguide.
In an alternative preferred embodiment, the modulating electrode may be provided so as to modulate the refractive index of the portion of the optical waveguide, to which the modulating electric field is applied, and may function as a phase modulator for modulating the phase of light that has been input to the optical waveguide.
In another preferred embodiment, the optical waveguide preferably includes at least two portions exhibiting remnant polarizations with mutually opposite polarities.
In still another preferred embodiment, the optical waveguide is preferably provided in a substrate that is made of the electro-optic material.
In yet another preferred embodiment, the electric signal input section preferably includes an input line, which forms another microstrip line with the conductive layer, and the input line is preferably connected to one of the first and second conductor lines.
In yet another preferred embodiment, the electric signal input section preferably includes: a coaxial connector, which is connected to a line that propagates the RF modulating signal therethrough; and an interconnecting member, which electrically connects the coaxial connector and the modulating electrode together.
An optical modulator according to another preferred embodiment of the present invention preferably includes an optical waveguide, a modulating electrode, a conductive layer and an electric signal input section. At least a portion of the optical waveguide is preferably made of an electro-optic material. The modulating electrode preferably includes a first conductor line and a second conductor line, which are coupled together electromagnetically, and preferably applies a modulating electric field to a portion of the optical waveguide. The conductive layer preferably forms a first microstrip line with the first conductor line and a second microstrip line with the second conductor line, respectively. Through the electric signal input section, an RF modulating signal is preferably supplied to the modulating electrode. In this optical modulator, the optical waveguide preferably includes at least two portions exhibiting remnant polarizations with mutually opposite polarities, and the first and second conductor lines preferably function as an odd-mode resonator for the RF modulating signal.
In one preferred embodiment of the present invention, the optical waveguide preferably includes: at least two optical waveguide branches; an optical input portion, which combines the two branches together; and an optical output portion, which also combines the two branches together. The portion of the optical waveguide, to which the modulating electric field is applied, is preferably divided into the two optical waveguide branches. The first and second conductor lines are preferably provided so as to apply electric fields with mutually opposite polarities to the two optical waveguide branches, respectively, and preferably function as an intensity modulator for modulating the intensity of light that has been input to the optical waveguide.
In an alternative preferred embodiment, the modulating electrode may be provided so as to modulate the refractive index of the portion of the optical waveguide, to which the modulating electric field is applied, and may function as a phase modulator for modulating the phase of light that has been input to the optical waveguide.
In another preferred embodiment, the optical modulator preferably further includes a connector member, which connects the first and second conductor lines together on at least one end thereof.
In still another preferred embodiment, the optical waveguide is preferably provided in a substrate that is made of the electro-optic material.
In yet another preferred embodiment, the electric signal input section preferably includes an input line, which forms another microstrip line with the conductive layer, and the input line is preferably connected to one of the first and second conductor lines.
In yet another preferred embodiment, the electric signal input section preferably includes: a coaxial connector, which is connected to a line that propagates the RF modulating signal therethrough; and an interconnecting member, which electrically connects the coaxial connector and the modulating electrode together.
An optical modulator according to still another preferred embodiment of the present invention preferably includes an optical waveguide, a modulating electrode, a conductive layer and an electric signal input section. At least a portion of the optical waveguide is preferably made of an electro-optic material. The modulating electrode preferably includes a first conductor line, a second conductor line and a third conductor line, which are coupled together electromagnetically, and preferably applies a modulating electric field to a portion of the optical waveguide. The conductive layer preferably forms a first microstrip line with the first conductor line, a second microstrip line with the second conductor line, and a third microstrip line with the third conductor line, respectively. Through the electric signal input section, an RF modulating signal is preferably supplied to the modulating electrode.
In one preferred embodiment of the present invention, the optical waveguide preferably includes: at least two optical waveguide branches; an optical input portion, which combines the two branches together; and an optical output portion, which also combines the two branches together. The portion of the optical waveguide, to which the modulating electric field is applied, is preferably divided into the two optical waveguide branches. The first and second conductor lines are preferably arranged so as to apply electric fields with mutually opposite polarities to one of the two optical waveguide branches. The second and third conductor lines are preferably arranged so as to apply electric fields with mutually opposite polarities to the other optical waveguide branch. The modulating electrode preferably functions as an intensity modulator for modulating the intensity of light that has been input to the optical waveguide.
In an alternative preferred embodiment, the modulating electrode may be provided so as to modulate the refractive index of the portion of the optical waveguide, to which the modulating electric field is applied, and may function as a phase modulator for modulating the phase of light that has been input to the optical waveguide.
In another preferred embodiment, the optical modulator preferably further includes a connector member, which connects the first, second and third conductor lines together on at least one end thereof.
In still another preferred embodiment, the optical waveguide preferably includes at least two portions exhibiting remnant polarizations with mutually opposite polarities.
In yet another preferred embodiment, the optical waveguide is preferably provided in a substrate that is made of the electro-optic material.
In yet another preferred embodiment, the electric signal input section preferably includes an input line, which forms another microstrip line with the conductive layer, and the input line is preferably connected to one of the first and third conductor lines.
In yet another preferred embodiment, the electric signal input section preferably includes: a coaxial connector, which is connected to a line that propagates the RF modulating signal therethrough; and an interconnecting member, which electrically connects the coaxial connector and the modulating electrode together.
A communications system according to yet another preferred embodiment of the present invention preferably includes the optical modulator according to any of the preferred embodiments of the present invention described above, an input section for inputting light to the optical modulator, and a control section for supplying the RF modulating signal to the optical modulator.
Other features, elements, processes, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.